Big maggots in your compost? They're soldier fly larvae

Compost bins. Photo: Oregon State University, EESC

Last Updated:

June 29, 2005

EUGENE – Most people shudder when they see maggots in their bin composter or compost pile. Don't be grossed out – they won't hurt you. In fact, these larvae play a role in breaking down and recycling nutrients back into the soil.

These maggots may actually be the larvae of "compost-dwelling soldier flies," according to Cindy Wise, compost specialist volunteer coordinator with the Lane County office of the Oregon State University Extension Service.

This European insect (scientific name Hermetia illucens) seems to be found everywhere in North America where people reside. They are especially common where flies can get to a wet, rotting food source, garbage or other unsanitary conditions.

In the "old" days, these large flies and their larvae were found commonly in slum, emergency hospital, army triage and unchanged diaper conditions.

Young soldier fly larvae are a gray-white color, segmented, about an inch in length and very active. As they mature they turn a dark brown color. They are torpedo-shaped and flattened, with tough-looking skin. The larval head is small and narrower than the body and the body bears no legs or other features except hairs and spines. The rear end of the body is blunt and bears breathing pores (spiracles).

Adult compost-dwelling soldier flies are black and about 5/8-inch long. They have smoky black wings. Their wings are held over the back when at rest. The first abdominal segment has clear areas. Adults feed and lay eggs on food waste (such as in a composter or uncovered compost pile), especially where conditions are moist.

The adults will emerge, mate and die in two days. The adult flies are black and often are mistaken for black wasps, said Wise. They do not bite or carry disease, as they have no hair on their legs.

"Don't worry, soldier flies don't usually invade houses, unless your compost pile is close to your house," said Wise. "They almost exclusively populate compost bins or sheet mulch compost piles and manure piles," she said. "In the southern United States they are being utilized to reduce hog manure, as they can consume up to 30 tons of hog manure in two days."

Soldier fly females lay eggs on the surface of nitrogen-rich material that is exposed. So, if you want to avoid having these large flies and their maggots in your compost pile, make sure you have enough leaves, dry grass, shredded paper and other organic "brown" material in the pile to cover the nitrogen food sources by at least two to four inches. Be sure to bury food scraps deeply in the pile and cover them well.

You can further discourage these flies by putting window screen over any holes in the bin and gluing it down with a waterproof caulking (like an exterior household caulk) on the inside of the bin to help exclude the flies in their egg laying stage.

They often thrive in worm bins, as well as compost bins, where they may out-compete the worms for food.

"In a worm bin, bury food scraps down at least six inches for the worms and let the flies eat what is on the surface," said Wise. "The flies don't eat the worms or their eggs so they aren't predators of the worms."

If you aren't too grossed out by the maggots, you can feed them to wild birds, suggested Wise.

"Birds love soldier fly larvae so you can remove them and feed them to chickens or just toss them on the ground and other birds may find them," she said. "They are actually very high in protein."

Wise and her colleagues are experimenting with soldier flies in compost bins and then analyzing the resulting compost to see what differences there may be in the nutritional content of the compost.

The maggots are known to break down organic material in the pile so it can further decompose. And the flies inoculate the compost with beneficial bacteria from other sources.